1. Field of the Invention
The present invention relates generally to an MPX stereo signal demodulator, and is directed more specially to an MPX stereo signal demodulator suitable for being made as an integrated circuit (IC).
2. Description of the Prior Art
Product demodulators have been used heretofore to obtain stereo audio signals from a composite stereo signal and a sub-carrier signal. The composite stereo signal, according to the standards used by the industry, includes a low frequency portion not modulated on a carrier and comprising the sum of the left (L) and right (R) electronic signals. This L+R signal extends from the lowest audio frequencies up to approximately 15 KHz. A difference signal L-R is modulated on a 38 KHz carrier by suppressed carrier modulation so that only the two side bands are left. These side bands are added to the low frequency sum signal L+R to form the complete composite stereo signal.
In one form of circuit for separating the L and R components from each other, the composite signal is applied to a differential amplifier that has a variable resistor in series with the emitter circuits of the differentially connected transistors. This variable resistor is connected so that emitter current of both of the transistors flows through the resistor to ground. This variable resistor is used as a separation control for controlling the amount of the L+R sum signal mixed with the demodulated L-R signals.
The demodulated L-R signals are obtained by two other differential amplifiers each of which is connected in series with the collector of a respective one of the differentially connected transistors in the first differential amplifier. The 38 KHz carrier signal is differentially applied to the pair of differential amplifier collector loads to render the transistors in the latter two differential amplifiers alternately conductive. The resultant demodulated L-R and R-L signals are combined with the L-R signal to obtain separated L and R signals at two output terminals of the circuit.
The foregoing prior art circuit has the disadvantage that variation of the resistance of the variable resistor in series with the emitters of both of the differentially connected transistors causes the DC level of the stereo demodulated output signal to fluctuate.
In another type of prior art circuit, there is a generally similar connection of the composite stereo signal to a differential amplifier that has a constant current circuit in the common emitter circuit thereof. Second and third differential amplifiers are connected in the collector circuits of the differentially connected transistors in the first differential amplifier in a manner similar to the previously described prior art circuit. A separate pair of transistors having their input circuit connected in parallel are also connected to receive the composite stereo signal. The variable resistor that operates as a separation control is connected in the common emitter circuits of the parallel connected transistors, and the collectors of the parallel connected transistors are connected to the output terminals of the second and third differential amplifiers to effect a multiplexing operation whereby separated L and R signals are obtained at the two output terminals of the circuit. This circuit still has the disadvantage that the DC level of the stereo modulated output signal fluctuates with variations of the separation control resistor.
Still another prior art circuit utilizes the basic stereo signal demodulator of one of the first two types of circuits but connects the two output signals to separate transistors that have the final output terminals at their collectors. The emitters of this pair of output transistors are connected to ground through separate resistors and are connected together through the variable separation control resistor. Variation of the resistance of the latter resistor does not change the DC level, but it is necessary to provide two IC terminals so that the two ends of the variable resistor can be connected between them. In the two first mentioned circuits, it was only necessary to provide a single IC terminal for connection to one end of the variable resistor. It is well known in IC technology that it is highly desirable to limit the number of terminals required for the IC device.